A wide variety of physiological transformations, chemical reactions, and technical processes produce or consume O2, while many anoxic species, numerous chemical syntheses, and manufacturing protocols demand its complete absence. The presence of O2 in an extraterrestrial environment is used to indicate the potential for life forms. Hence, trace oxygen detection is important for many endeavors, including aerospace research, environmental safety and food storage.
Common trace oxygen sensors are based on amperometry (Clark electrodes). Although amperometry based instruments are sensitive and applicable over a wide temperature range, they are difficult to miniaturize, invasive, limited to discrete positions, and limited by the stability of the electrode surface and the oxygen diffusion barrier at the surface. To overcome such problems, attention has been directed to optical sensing methods based on luminescence quenching of an indicator by oxygen. Most optical oxygen sensors are based on the quenching of the long-lived luminescence exhibited by polycyclic aromatic hydrocarbons, transition-metal complexes, and metalloporphyrins. Typically, these compounds are placed in inert permeable polymer membranes.
The highest optical oxygen sensitivity has been achieved by detecting the quenching of the delayed fluorescence of fullerenes embedded in permeable polymeric films. The electronic states and transitions of fullerenes are near the interface between discrete molecular orbitals and band structures due to the large number of electrons in π orbitals. The fluorescence of fullerenes, such as C60 and C70, is atypical in several ways and has been exploited to produce oxygen sensors that have sensitivities in the parts per billion range as reported in: Amao et al. (“Optical Sensor System Using Photofunctional Materials for Oxygen Pressure on Solid Surface.” Analyst 2000, 125:523-6) where fluorescence decay was monitored at 740 nm; and Nagl et al. (“Optical Sensing and Imaging of Trace Oxygen with Record Response.” Angew. Chem, Int. Ed. 2007, 46:2317-9) where emission was monitored at 650 to 710 nm. These sensors were determined to be stable over multiple (>100) light exposures in oxygen containing atmospheres with no disclosed degradation of the fullerenes.
However, the insolubility of fullerenes in many solvents has limited their application and production for uses such as oxygen detection sensor. New materials with good oxygen sensitivity while being environment friendly and easy to manipulate are highly desired.